Microwave oven leakage detection system

ABSTRACT

An electronic microwave oven has incorporated therewithin a leakage radiation detecting device for sensing and providing an indication of the presence of radiation escaping from the oven cavity past the door seal. The device, located centrally within a collection cavity waveguide formed within the hollow oven door, includes an antenna probe projecting into the collection cavity waveguide coupled with the appropriate amplification circuitry and a neon lamp. Elongated coupling slots are provided on each side of the door located in the channel-like area external to the door seal, each slot having a length of approximately one-half the wavelength of the operating frequency of the microwave oven. Any microwave radiation escaping past the door seal is coupled through the slots into the radiation collection cavity within the door and is sensed by the antenna probe. The amplification circuitry is set to cause the neon lamp to be activated when the radiation at the antenna probe reaches a predetermined level.

United States Patent 1 Fitzmayer et al.

I 1111 3,749,875 1451 July 31,1973

[ MICROWAVE OVEN LEAKAGE DETECTION SYSTEM Inventors: Louis H. Fitzmayer, Louisville;

Nelson B, Worden, Jeffersontown, both of Ky.

General Electric Company, Louisville, Ky.

Filed: Aug. 7, 1972 Appl. No.: 278,595

[73] Assignee:

US. Cl. 219/1055, 324/122 110. c1. H05b 9/06 Field of Search 219/1055; 324/95,

References Cited OTHER PUBLICATIONS Movchan, Zatenko, and Kononenko, Waveguide Gas-Discharge Indicator of Flowing Microwave Power,"'Radio' Engineering and Electronic Physics, Vol. 15, No. 3, 1970.

Primary Examiner-J. V. Truhe Assistant ExaminerHugh D. Jaeger Attorney-Richard L. Caslin et al.

[57] ABSTRACT An electronic microwave oven has incorporated therewithin a leakage radiation detecting device for sensing and providing an indication of the presence of radiation escaping from the oven cavity past the door seaL-The: device, located centrally within 'a collection cavity wavelength ofthe operating frequency of the microwave oven. Any microwave radiation escaping past the door seal is coupled through the slots into the radiation collection cavity within the door and is sensed by the antenna probe. The amplification circuitry is set to cause the neon lamp to be activated when the radiation at the antenna probe reaches a predetermined level.

7 Claims, 2 Drawing Figures PATENIED un. 1 ms MICROWAVE OVEN LEAKAGE DETECTION SYSTEM BACKGROUND OF THE INVENTION .extreme care in the design of the oven to insure the safety of the user against leakage of microwave radiation, particularly around the oven door seal. However, despite the precautions taken by the manufacturer, it is sometimes possible for an oven to become leaky dur-. ing its lifetime due to structural damage of the door seal gasket or due to accumulation of spilled food particles on the door seal. For the safety of the user, the Department of Health, Education and Welfare/Bureau of Radiological Health regulations specify that the maximum allowable leakage from any oven surface, other than in-factory meansurements, be 5 milliwatts per square centimeter measured 5 centimeters from any oven surface. I

A reliable device is needed, therefore, to sense and give to the operator an indication of possible leakage radiation at such a low level. One device is described in U. S. Pat. No. 3,436,508-Fritz wherein a pair of conductors are located on the oven cabinet just beyond the outer edge of the door opening and looped therearound and which are connected to an alarm means for announcing the presence of leakage radiation. Such a device, however, has certain disadvantages, the most important of which is lack of sensitivity. With conductor wires mounted in such a fashion, there is very little possibility of their being responsive to the electric field of the radiation since they generally would not be'parallel with this electric field. Neither would they be responsive to the magnetic field of the radiation, the net effect 7 upon the wires in this case being normally close to zero.

It is therefore. the principle object of this invention to provide a device built into a domestic microwave oven for sensing and indicating the presence of microwave radiation leaking from the oven cavity past the door seal, a device that is sensitive to such low level leaks as mentioned above and that is simple and reliable in operation.

SUMMARY OF THE INvENTIoN- BRIEF DEscRIPTIoN OF THE DRAWINGS In the accompanying drawings:

FIG. 1 is a perspective view of a typical domestic freestanding electric range including an electronic microwave oven; and

FIG. 2 is an enlarged fragmentary view, partly in section, of a side elevation, taken with the access door in the closed position, of a portion of an electronic microwave oven incorporating a preferred embodiment of the present invention.

, DESCRIPTION OF THE PREFERRED EMBODIMENT Referring generally to FIGS. 1 and 2 of the drawing, there is illustrated a household electric range 10 embodying the features of the present invention, the range 10 including an electronic microwave oven 11.,More

particularly, range 10 icom'prises'an upstanding; substantially box-like metal body 12' carrying a' substantially horizontal metal cooking top 13 and an upstanding metal backsplash 14 arranged at the rear of the cooking top 13. The body 12 constitutes a shell or casing and includes a door frame such as frontdoor frame 15, an outer rear wall (not shown) and a pair of outer side walls 16. Also, body 12 comprises a supporting base and bottom wall (not shown) defining a lower machinery compartment in which the microwave energy supply means may be located.

A substantially box-like metallic .oven liner 20 is housed in the upper portion of the body 12 and, in conjunction with metallic access door 25, defines a microwave oven cooking cavity 26 therein. Liner 20 includes a rear wall 27, a top wall 28 (shown partially in FIG. 2), a bottom wall 29 and a pair of side walls 30. The forward edges of the top wall 28, the bottom wall 29 and the side walls 30 define an opening into the cooking cavity 26, there being a continuous outwardly facing flange 31 which is bent back upon itself on the rear surface thereof as shown. The front door frameils has an opening therein slightly greater than the area defined by the outer edges of the flange 31 and has a rearwardly directed flange 32 which is suitably secured to the several 'walls of the, oven liner20, as by welding, so as to interconnect the front door frame 15 and the oven liner Front opening access door 25 is mounted on the front of the body 12 by hinge structure 33 and is movable about the lower edge thereof between a substantially horizontal open position (shown in FIG. 1) and a substantially vertical closed position with respect to the open front of the oven cavity 26 as illustrated in FIG. 2.

Referring now to FIG. 2, access door 25 more particularly includes a metal outer sheet 34, a metal inner sheet 35 spaced well inwardly of the outer sheet 34,

and a metal intermediate sheet 36 disposed between the outer sheet 34 and the inner sheet 35. The outer.

sheet 34 carries an intumed flange 37 around the peripherythereof and is suitably attached to intermediate sheet 36,flange 37 extending inwardly toward and to a point spaced a short distance from the front door frame 15 when the access door 25 is in its closed position. The inner sheet 35 has a forwardly directed flange 38 that is suitably connected to intermediate sheet 36, as by welding. As illustrated, flange 38 forms a portion of the inner surface of the door 25 and is disposed substantially parallel to and spaced a short distance from a portion of the flange 32 so as to define a channel-like space such as continuous channel 40 which extends between intermediate sheet 36'and door frame 15.

Microwave radiation sealing means such as metallic radiation seal gasket 45 is fixedly attached to inner sheet 35 of door 25 and provides a seal and an electrical connection between flange 31 and the inner surface of the door 25 to contain the microwave energy within the oven cavity during the cooking process. Gasket 45 may be of the type described in U.S. Pat." No. 3,459,92l-Fussell, and assigned to the same assignee as the present invention.

In accordance with the present invention, a leakage radiation monitor 50 is provided for detecting the presence of excess microwave radiation leakage past door seal 45. To this end there is provided a hollow space in the access door 25 between sheets 35 and 36 which comprises a leakage radiation collection cavity 54. To the extent there may be leakage of microwave energy past the door seal, means including elongated slots 56 are provided for coupling at least a portion of this leakage radiation into the collection cavity 54 in the interior of door 25. In the embodiment shown in FIG. 1, three slots 56 are spaced along each of the top and bottom sides of door 25 while one slot is located on each of the vertical sides of the door 25. The slots 56 are located on flange 38 of door 25. Each coupling slot 56 has a length approximately equal to one-half wavelength of the operating microwave excitation frequency which for 915 MHz is approximately equal to 16.5 cm. or 6% inches, and each slot has a width of approximately 0.45 cm. or three-sixteenths inches. The length of slots 56 may be varied but preferably should be whole multiples of one-half wavelength of the excitation frequency.

Monitor 50 also includes means responsive to the microwave energy coupled through slots,56 into collec tion cavity 54 within door 25 for providing an indication of any excess microwave radiation leakage past door seal 45. This indication means includes an antenna probe 51 extending into collection cavity 54 at a central location on door 25 vertically and horizontally. The voltage produced in 51 by the microwave energy is coupled to an indicator such as neon lamp 53 via an amplifier circuit 52. Circuit 52 is a conventional amplifier circuit suitably designed such that a voltage in probe 51 above a predetermined level corresponding to radiation leakage past door seal 45 in excess of permissible levels to cause lamp 53' to be lighted thus indicating the need to seek-necessary repair to the seal.

More specifically, in a situation where microwave radiation leaks from cooking cavity 26 past seal gasket 45 at any location on the periphery thereof, at least one coupling slot and thereby collection cavity waveguide 54 within access door 25 are caused to be excited. The size of collection cavity 54 is such that a leak anywhere along the top of seal gasket 45 will cause one of the coupling slots 56 to excite cavity 54 in a mode propagating a transverseelectric wave from top to bottom with one one-half sine wave loop of field variation and with three half sine wave loops of field variation'from side to side. Assigning the sides conventional-directions X, Y and Z, with Z in this case beingthe direction of propagation, and assigning thev convention for a transverse electric wave as TE in the case of a leak pro'pagating from top to bottom within waveguide 54, the result is a transverse electric wave TE A leak along the side excites a transverse electric wave propagating from side to side within waveguide 54 with three onehalf sine wave loops of field variation and taking as convention TE a transverse electric wave 'I'E results. For developmental purposes the dimensions of waveguide 54 within door 25 were approximately 21 inches wide by 15% inches high. These dimensions sup- I ported such transverse electric waves at 915 MHz.

It will be seen then that the electric field is at a maximum in the center of this cavity and it is for this reason that the antenna probe 51 projects across cavity 54 at its center.

When the collection cavity 54 is excited, whether the antenna probe means be one sensitive to the electric field or to the magnetic field, a voltage will appear within the probe, which voltage will be amplified by the amplifier circuit 52 so as to activate indicator 53, as previously described, thus serving to warn the operator of the presence of leakage radiation.

It will be appreciated that the indication means may also comprise an audible alarm such as a buzzer which will likewise be activated when radiation above a predetermined level is present. it is also within the scope of the invention that the indication means may include suitable conventional circuitry to respond to the voltage from probe 51 to deactivate the principle microwave source in the oven. For example, a relay or circuit breaker type of device could be used to sever the supply of line voltage to effectively inactivate the microwave oven.

It should be apparent to those skilled in the art that the embodiment described heretofore is considered to be the presently preferred form of this invention. In accordance' with the Patent Statutes, changes may be made in the disclosed apparatus and the'manner in which it is used without actually departing from the true spirit and scope of this invention.

What is claimed is:

l. A microwave oven radiation leakage monitor comprising:

an hollow oven access door having microwave sealing means between the door and the body of the oven;

means for coupling at least a portion of any microwave radiation escaping past the door seal into the interior of the access door; and

means responsive to said coupled microwave energy within the door for providing an indication of excess microwave radiation leakage past the oven door seal.

2. The monitor of claim 1 in which said energy responsive means includes an antenna probe projecting into the interior of said door at approximately the center thereof.

3. The monitor of claim 1 in which said coupling means comprises a plurality of elongated slots in the door surface adjacent the exterior side of the door seal.

4. The monitor of claim 3 in which the elongated slots are on opposing sides of the door surface.

5. in a microwave oven of the type having a metallic box-like oven liner and a hollow metallic access door defining an oven cooking cavity, and microwave radiation sealing means between thedoor andoven liner, a leakage radiation detecting monitor comprising:

a leakage-radiation collection-cavity waveguide formed within the hollow access door;

at least one elongated coupling slot located on each side of the hollow access door external to the radia- I tion sealing means and opening into the collection cavity waveguide, each slot having a length approximately equal to whole multiples of one-half wavelength of the operating microwave frequency;

6. The monitor of claim 5 wherein the probe means projects into the collection cavity waveguide at approximately the center thereof. l

l 7. The monitor of claim 5 wherein three such elongated coupling slots are located respectively on the top and bottom sides of the door and one such slot is located respectively on each of the remaining sides ofthedoor.

*a: a a: 

1. A microwave oven radiation leakage monitor comprising: an hollow oven access door having microwave sealing means between the door and the body of the oven; means for coupling at least a portion of any microwave radiation escaping past the door seal into the interior of the access door; and means responsive to said coupled microwave energy within the door for providing an indication of excess microwave radiation leakage past the oven door seal.
 2. The monitor of claim 1 in which said energy responsive means includes an antenna probe projecting into the interior of said door at approximately the center thereof.
 3. The monitor of claim 1 in which said coupling means comprises a plurality of elongated slots in the door surface adjacent the exterior side of the door seal.
 4. The monitor of claim 3 in which the elongated slots are on opposing sides of the door surface.
 5. In a microwave oven of the type having a metallic box-like oven liner and a hollow metallic access door defining an oven cooking cavity, and microwave radiation sealing means between the door and oven liner, a leakage radiation detecting monitor comprising: a leakage-radiation collection-cavity waveguide formed within the hollow access door; at least one elongated coupling slot located on each side of the hollow access door external to the radiation sealing means and opening into the collection cavity waveguide, each slot having a length approximately equal to whole multiples of one-half wavelength of the operating microwave frequency; antenna probe means projecting into the collection cavity waveguide for sensing microwave radiation appearing therein; and means coupled to the antenna probe and responsive to energy sensed by the probe for providing an indication of microwave radiation leakage past the door seal that exceeds a predetermined level.
 6. The monitor of claim 5 wherein the probe means projects into the collection cavity waveguide at approximately the center thereof.
 7. The monitor of claim 5 wherein three such elongated coupling slots are located respectively on the top and bottom sides of the door and one such slot is located respectively on each of the remaining sides of the door. 